Voyager 1 had as its primary targets the planets Jupiter and Saturn and their associated moons and rings; its current mission is the detection of the heliopause and particle measurements of solar wind and the interstellar medium. Both Voyager probes are powered by three radioisotope thermoelectric generators, which have far outlasted their originally intended lifespan, and are now expected to continue to generate enough power to keep communicating with Earth until around the year 2020.

Mission planning and launch

Voyager 1 was originally planned as Mariner 11 of the Mariner program. From the outset, it was designed to take advantage of the then-new technique of gravity assist. By fortunate chance, the development of interplanetary probes coincided with an alignment of the planets called the Grand Tour. The Grand Tour was a linked series of gravity assists that, with only the minimal fuel needed for course corrections, would enable a single probe to visit all four of the solar system's gas giantplanets: Jupiter, Saturn, Uranus, and Neptune. The identical Voyager 1 and Voyager 2 probes were designed with the Grand Tour in mind, and their launches were timed to enable the Grand Tour if desired.

Initially, an underburn in the second stage of the Titan IIIE rocket left an estimated one second worth of fuel remaining in that stage. Although ground crews were worried that Voyager 1 would not make it to Jupiter, the Centaur upper stage proved to have enough fuel to compensate.

For details on the Voyager instrument packages, see the separate article on the Voyager program.

Jupiter

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Detail of Jupiter's atmosphere, as imaged by Voyager 1.

Voyager 1 began photographing Jupiter in January 1979. Its closest approach to Jupiter was on March 5, 1979, at a distance of 349,000 kilometers (217,000 miles) from its center. Due to the greater resolution allowed by close approach, most observations of the moons, rings, magnetic fields, and radiation environment of the Jupiter system were made in the 48-hour period bracketing closest approach. It finished photographing the planet in April.

The two Voyager spacecraft made a number of important discoveries about Jupiter and its satellites. The most surprising was the existence of volcanic activity on Io, which had not been observed from the ground or by Pioneer 10 or 11.

Saturn

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Voyager 1 image of Saturn from 5.3 million km four days after its closest approach.

The gravity assist at Jupiter was successful, and the spacecraft went on to visit Saturn. Voyager 1's Saturn flyby occurred in November 1980, with the closest approach on November 12 when it came within 124,000 kilometers (77,000 miles) of the planet's cloud-tops. The craft detected complex structures in Saturn's rings, and studied the atmospheres of Saturn and Titan. Because of the earlier discovery of a thick atmosphere on Titan, the Voyager controllers at the Jet Propulsion Laboratory elected for Voyager 1 to make a close approach of Titan and terminate its Grand Tour. (For the continuation of the Grand Tour, see the Uranus and Neptune sections of the Voyager 2 article.) The Titan-approach trajectory caused an additional gravity assist that took Voyager 1 out of the plane of the ecliptic, thus ending its planetary science mission.

Interstellar mission

It is estimated both Voyager craft would have sufficient electrical power to operate at least some instruments until 2020.

Heliopause

As the Voyager 1 space probe heads for interstellar space, its instruments continue to study the solar system; Jet Propulsion Laboratory scientists are using the plasma wave experiments aboard Voyager 1 and 2 to look for the heliopause.

Scientists at the Johns Hopkins University Applied Physics Lab believe that Voyager entered the termination shock in February 2003. Some other scientists have expressed doubt, discussed in the journal Nature of November 62003. In a scientific session at the American Geophysical Union meeting in New Orleans on the morning of March 252005, Dr. Ed Stone presented clear evidence that Voyager 1 crossed the termination shock in December 2004 [1] (http://www.agu.org/cgi-bin/SFgate/SFgate?&directget=1&application=sm05&database=%2Fdata%2Fepubs%2Fwais%2Findexes%2Fsm05%2Fsm05&=%22SH22A-01%22). The issue will not be resolved for some months as other data become available, since Voyager's solar-wind detector ceased functioning in 1990. However, in May2005 a NASA press release said that consensus was that Voyager 1 was now in the heliosheath. [2] (http://www.nasa.gov/vision/universe/solarsystem/voyager_agu.html).

Distance travelled

In March 2005, Voyager 1 was at a distance of 14.2 billion kilometers (95.0 AU or 8.83 billion miles) from the Sun, which makes it the most distant man-made object from Earth. It was travelling at a speed of 17.2 kilometers per second (3.6 AU per year or 38,400 miles per hour), 10% faster than Voyager 2. It is not heading straight towards any particular star, but even if Voyager 1 were going straight toward the closest star system, Alpha Centauri, it would take about 80,000 years to get there.